unix: add gccgo support

With these changes the package builds, and the tests pass, using gccgo
on amd64 GNU/Linux.  I have not tested other systems but I don't know
why they wouldn't work.

Change-Id: I727365daef55f158657eb89afbfcdbf3334610d1
Reviewed-on: https://go-review.googlesource.com/9842
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/unix/asm_linux_386.s b/unix/asm_linux_386.s
index 5d3ad9a..4db2909 100644
--- a/unix/asm_linux_386.s
+++ b/unix/asm_linux_386.s
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !gccgo
+
 #include "textflag.h"
 
 //